1. Field of the invention.
The present invention relates to a method and apparatus for damping oscillations of rotating rolls in a paper machine.
2. Description of the related art.
The web being produced travels in a paper machine across a plurality of rolls. At least one pair of rolls forms a nip through which the web passes, for example, in the press section, in coaters, winders or smoothing presses. Here, the problem of contact oscillations occurs persistently.
The nature of contact oscillations is that the axes of the two nip-forming rolls move during operation relative to each other, thus unintentionally deforming. This problem increases the higher the web velocity (partly over 2,000 m/min) and the wider the web and, hence, the wider the machine with its respective rolls.
At web velocities in excess of 1,000 m/min and with extremely large web widths, which may measure up to 10 meters, the intensity of the oscillations is such that the rolls deform unevenly (polygonally) and wear. In addition, the oscillations result in a degradation in the operation of the machine. In coaters, the length profile quality of the applied coating undergoes adulteration due to the oscillations.
It is known already to dampen roll oscillations or flexures passively. This is done, e.g., by creating a specially configured flow cross section of a pressure space in the stationary central axis of a rotatable wall shell, as taught in German Document No. 2950945.
Known from US Pat. No. 5,431,261 is a method for damping oscillations of a large mass. This method employs an antivibration device with an additional mass, the latter counteracting the mass to be damped.
The German patent application DE 196 35 216 describes a method and a winder, for winding a paper web into a roll, featuring active oscillation damping. The winder includes an antivibration device with an additional damping mass which acts on the rider roll of the paper roll. The antivibration device includes at least one actuator operating hydraulically or pneumatically. The actuator generates phase-shifted oscillations, thereby extensively suppressing the oscillations of the rider roll.
The present invention provides a method and apparatus capable of reliably eliminating, or at least damping, contact oscillations of nip-forming rolls. An additional mass such as in the prior art is not to be used in this concept.
The inventors recognized that an active damping of the oscillations of coacting, i.e., nip-forming, rolls can be achieved only by active outside stimulation directed at at least one of the bearings of a roll (tending side and/or gear side). The forces being introduced may also act on the journal of the bearing arrangement, outside or within the bearing point. A sensor attached to the bearing point(s) can measure the roll oscillations and forward an active damping command via an actuator.
It is also possible to position the sensor in the machine center.
Employed for active damping are elements that allow an automatic adaptation to varied conditions. This is very important in order to be able to react correctly and swiftly to changing conditions of production (for example, speed) or machine conditions (e.g., aging roll covering with the associated elastic properties).
Surprisingly and unexpectedly it has been found that an active and effective countermeasure is possible on at least one bearing point, despite the oscillation node being situated there.
The counteroscillation is a sinusoidal oscillation. But it may also have a pulse-like square wave oscillation characteristic (rectangular oscillation). The counteroscillation need not act on every amplitude, but only, e.g., on each second, third, or fourth, etc.
The present invention relates to a paper machine which may be either a paper-making machine or an off-line coater.